The wide variety of uses for optical waveguide fiber dictates a corresponding variety of packaging for the waveguide. Perhaps the most familiar package is the optical waveguide cable commonly used for telecommunications, network and data applications. In these cases, the waveguide is usually deployed linearly, to cover the greatest distance with the smallest number of meters of waveguide.
However, for certain sensor applications, such as gyroscopes, for various tethered, remotely controlled devices, such as missiles, and for specialty applications, such as dispersion compensating waveguide fiber or delay lines, the desired package takes a much different form. In these applications, a length of optical waveguide fiber is wound onto a bobbin in a configuration usually comprising layers of fiber. This configuration is termed a filament pack. The filament pack must be wound such that key fiber properties, e.g., attenuation and fatigue resistance, are not compromised. Furthermore, fiber properties must remain essentially unchanged for extended periods of time over a range of environmental conditions including, temperature and humidity excursions, vibration and rough handling.
The stability of the filament pack is especially important in applications where the fiber must be unwound after a period of storage. For example, in a tethered weapon application, the filament pack remains on the bobbin for an indefinite period of time, and may be located on an aircraft or land vehicle and thus subjected to abrupt accelerations and wide ranges of temperature and humidity. Also, in tethered missile applications, unwinding speeds can be more than 200 meters per second.
It has been found that stability of waveguide fiber optical and mechanical properties is improved when a base layer is wound on the bobbin base, i.e., the elongated portion of the bobbin which supports the filament pack, prior to winding the filament pack. A base layer is the first layer to be wound on the bobbin base and serves to control spacing and pitch of the subsequent wound layers of the filament pack.
In the prior art a typical base layer is a series of contiguous coils of wire wound upon the bobbin surface. U.S. Pat. No. 4,995,698, Meyers, U.S. Pat. No. 4,957,344, Chesler et al., and U.S. Pat. No. 4,950,049, Darsey et al., are examples of bobbin winding art relating to a wire base layer used in conjunction with an optical waveguide filament pack. Because the base layer determines the pitch and spacing of the filament wound thereon, the base layer wire must have a diameter tolerance of about 3-6 microns. Further, the base layer winding must be uniform in tension, the coils must abut one another and the wire must be free from dirt particles. Since the wire winding tension is typically very different from the filament pack winding tension, a dedicated winding machine is usually required. Essentially continuous side pressure must be exerted on the wire during winding to insure close abutment of the wire coils. Also, the wire usually must be cleaned before winding. Further, the process of making precision diameter wire is costly. Thus, a strategy employing wire as a base layer is expensive in terms of capital, cash flow and labor.
The large difference in thermal coefficient of expansion, between wire and the filament wound thereon, may cause filament pack instability when the filament pack is subjected to temperature excursions. A typical temperature specification may cover the range -40.degree. to more than +65.degree. C.
To address these deficiencies, a compliant base layer which more nearly matches the properties of the filament buffer has been proposed. U.S. Pat. No. 5,029,960, Hulderman et al., is directed to a base layer of silicone. U.S. Pat. No. 5,125,590 teaches the use of an elastomer base layer together with spring loaded bobbin flanges which press against the filament pack.
However, in both of these strategies, additional steps are added to the bobbin preparation and winding process. Also, the base layer is smooth in both cases and so cannot serve as a template for pitch and spacing of the wound filament pack.
A further complication in developing a strategy to maintain filament stability is introduced if the polymer coating of optical fibers in the filament pack changes dimension after winding, perhaps due to the absorption or evolution of gaseous or liquid materials.
Therefore, there is a need for a base layer which isolates the filament pack from dimension changes of the bobbin, which matches the expansion coefficient of the filament pack, which serves as a template for spacing and pitch of the filament pack windings and which compensates for filament pack dimension changes due to absorption or desorption of gaseous or liquid material.